Diffuse radiation and its impact on photovoltaic power generation.
During the fall and winter, diffuse radiation has a significant impact on the power generation of a PV system. This article explains what diffuse radiation means and how high it can be in the United States.
What is diffuse radiation?
Diffuse radiation is solar radiation that does not reach the Earth’s surface directly from the Sun. The Sun’s rays are absorbed and scattered by clouds, air molecules, aerosols, and atmospheric particles before reaching the Earth. This scattering causes the Sun’s rays to reach the Earth’s surface at different angles.
Together with direct radiation, diffuse radiation forms global radiation.
What is the difference between diffuse and direct radiation?
Direct radiation reaches the Earth’s surface directly from the Sun, without scattering, and is more intense and concentrated compared to diffuse radiation.
How to measure diffuse radiation?
Pyranometers are used to measure global radiation resulting from a combination of direct and indirect solar radiation. By blocking direct solar radiation with a shadow ring or disk, a pyranometer can measure diffuse solar radiation that is scattered by air molecules, particles, and clouds.
How high is the proportion of diffuse radiation in the United States?
In the United States, diffuse radiation accounts for 50% of global radiation. In the summer, direct radiation dominates, while in the winter, it is mainly diffuse radiation that reaches the earth.
When the sky is clear, global radiation for the United States is about 1,000 W/m², most of which consists of direct radiation. As cloud cover increases, the proportion of diffuse radiation increases.
| Weather conditions | Summer | winter |
|---|---|---|
| Clear skies | 1,000 W/m² | 500 W/m² |
| Light Clouds | 600 – 800 W/m² | 300 – 400 W/m² |
| Light to moderate cloud cover | 300 – 600 W/m² | 150 – 300 W/m² |
| Thick clouds | 100 – 300 W/m² | 50 – 150 W/m² |
What factors determine diffuse radiation?
The proportion of diffuse radiation is determined by two influencing factors. In the long term, the location is decisive, because the closer you are to the equator, the stronger the global radiation. In the short term, diffuse radiation is determined by clouds, air molecules, aerosols, and atmospheric particles.
How does diffuse radiation affect the performance of photovoltaic power generation?
In general, diffuse radiation results in lower power generation from PV systems compared to clear sunny days with a higher proportion of direct radiation. However, in temperate climates with frequent clouds, production can still be high year-round.
The following points illustrate this impact:
Reduced intensity: Diffuse radiation is less intense than direct solar radiation. This means that less energy reaches the PV panels, resulting in lower power generation;
More evenly distributed: Diffuse radiation, on the other hand, is more evenly distributed, reaching the module from different directions. This can be beneficial for installations that are not optimally oriented toward the sun;
Some technologies perform better: Some PV technologies, such as thin-film cells, can respond to diffuse radiation better than traditional crystalline silicon cells. This means that these technologies perform relatively better on cloudy days.
Cooler temperatures: Cloudy days are generally cooler, which can improve the efficiency of PV panels, as high temperatures can affect their performance.
Because diffuse radiation is relatively high, PV systems in the United States are usually designed to take advantage of these conditions. This is done by choosing the right technology or adjusting the tilt and orientation of the solar modules.